The government says standards are being maintained
The Conservatives say they will make past GCSE and A-level exam papers available online so that standards can be tracked over time.
Tory education spokesman Michael Gove said public confidence in the exam system needed to be restored.
The Tories said they would also allow state schools to take the International GCSE (IGCSE) and new Pre-U exams - which they say are more challenging.
The government called the Tories' online archive idea a "gimmick".
People would be able to upload past exam papers onto the proposed internet library, which it says could stretch back to Victorian times.
Exams are becoming increasingly devalued, the Conservatives say, and an online archive would allow the public to scrutinise the difficulty of exams and any changes over time.
They say there are particular problems in GCSE maths and science, describing some of the questions posed in science papers as "ludicrous".
Increasing numbers of private schools are opting for International GCSEs in some or all subjects, because they say they are more stretching.
But in 2006 the then exams watchdog, the Qualifications and Curriculum Authority, judged the IGCSE "not suitable for assessing what children learn".
The IGCSE is not accredited for use in state schools because it does not cover the national curriculum - which is mandatory.
Shadow education spokesman Michael Gove said it was "vital" public confidence be restored in the exam system.
"Universities, businesses and academics say the system has been devalued and private schools are opting out of GCSEs for international exams," Mr Gove said.
"Now the government treats exam papers like state secrets and refuses to publish them.
"This is wrong and a Conservative government will create a free online library of all exam papers and scripts so there is full transparency and academic scrutiny of our exam system."
He said he believed head teachers should have the freedom to choose between the IGCSE and the usual GCSE.
This would not require an increase in funding, he added, as it would mean head teachers could choose how to spend their allotted examinations budget.
Children's Minister Dawn Primarolo said: "Parents want to know that standards are being maintained over time, which is why Ed Balls decided to establish Ofqual the new independent regulator of exam standards.
"Gimmicks like this from the Tories are all well and good, but what really counts is backing Ofqual to do their job."
However, the qualifications watchdog Ofqual said it had already proposed such an online archive.
Its chair, Kathleen Tattersall, said Ofqual wanted to publish both questions and answers online.
"We need to bring in front of the public eye the actual work that students do so they can see for themselves what demands are made on students who get high grades," she said.
A spokesman for exam board OCR said the board was in favour of creating a "national scripts archive" and allowing mark schemes, scripts, papers and information about grade boundaries to be made available.
He said the Tories' proposal was a positive idea, but providing exam papers on their own would not answer critics of exam standards.